Re: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Sep 09, 2019 at 11:28:04AM -0500, Mike Christie wrote:
> There are several storage drivers like dm-multipath, iscsi, and nbd that
> have userspace components that can run in the IO path. For example,
> iscsi and nbd's userspace deamons may need to recreate a socket and/or
> send IO on it, and dm-multipath's daemon multipathd may need to send IO
> to figure out the state of paths and re-set them up.
> 
> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
> memalloc_*_save/restore functions to control the allocation behavior,
> but for userspace we would end up hitting a allocation that ended up
> writing data back to the same device we are trying to allocate for.
> 
> This patch allows the userspace deamon to set the PF_MEMALLOC* flags
> through procfs. It currently only supports PF_MEMALLOC_NOIO, but
> depending on what other drivers and userspace file systems need, for
> the final version I can add the other flags for that file or do a file
> per flag or just do a memalloc_noio file.
> 
> Signed-off-by: Mike Christie <mchristi@redhat.com>
> ---
>  Documentation/filesystems/proc.txt |  6 ++++
>  fs/proc/base.c                     | 53 ++++++++++++++++++++++++++++++
>  2 files changed, 59 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 99ca040e3f90..b5456a61a013 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -46,6 +46,7 @@ Table of Contents
>    3.10  /proc/<pid>/timerslack_ns - Task timerslack value
>    3.11	/proc/<pid>/patch_state - Livepatch patch operation state
>    3.12	/proc/<pid>/arch_status - Task architecture specific information
> +  3.13  /proc/<pid>/memalloc - Control task's memory reclaim behavior
>  
>    4	Configuring procfs
>    4.1	Mount options
> @@ -1980,6 +1981,11 @@ Example
>   $ cat /proc/6753/arch_status
>   AVX512_elapsed_ms:      8
>  
> +3.13 /proc/<pid>/memalloc - Control task's memory reclaim behavior
> +-----------------------------------------------------------------------
> +A value of "noio" indicates that when a task allocates memory it will not
> +reclaim memory that requires starting phisical IO.
> +
>  Description
>  -----------
>  
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ebea9501afb8..c4faa3464602 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1223,6 +1223,57 @@ static const struct file_operations proc_oom_score_adj_operations = {
>  	.llseek		= default_llseek,
>  };
>  
> +static ssize_t memalloc_read(struct file *file, char __user *buf, size_t count,
> +			     loff_t *ppos)
> +{
> +	struct task_struct *task;
> +	ssize_t rc = 0;
> +
> +	task = get_proc_task(file_inode(file));
> +	if (!task)
> +		return -ESRCH;
> +
> +	if (task->flags & PF_MEMALLOC_NOIO)
> +		rc = simple_read_from_buffer(buf, count, ppos, "noio", 4);
> +	put_task_struct(task);
> +	return rc;
> +}
> +
> +static ssize_t memalloc_write(struct file *file, const char __user *buf,
> +			      size_t count, loff_t *ppos)
> +{
> +	struct task_struct *task;
> +	char buffer[5];
> +	int rc = count;
> +
> +	memset(buffer, 0, sizeof(buffer));
> +	if (count != sizeof(buffer) - 1)
> +		return -EINVAL;
> +
> +	if (copy_from_user(buffer, buf, count))
> +		return -EFAULT;
> +	buffer[count] = '\0';
> +
> +	task = get_proc_task(file_inode(file));
> +	if (!task)
> +		return -ESRCH;
> +
> +	if (!strcmp(buffer, "noio")) {
> +		task->flags |= PF_MEMALLOC_NOIO;
> +	} else {
> +		rc = -EINVAL;
> +	}

Really? Without any privilege check? So any random user can tap into
__GFP_NOIO allocations?

NAK.

I don't think that it's great idea in general to expose this low-level
machinery to userspace. But it's better to get comment from people move
familiar with reclaim path.

-- 
 Kirill A. Shutemov